The remote sensing of ocean color by use of a radiometric sensor on a satellite platform is a widely used technique to determine the concentrations of major seawater constituents in the ocean. These constituents are phytoplankton and their detritus, inorganic suspended material and dissolved organic matter. A remote spectral radiometer on a satellite platform is tuned to receive light at several wavelengths in the visible region and near-infrared portions of the electromagnetic spectrum. They are several pathways by which photons from the sun can enter the ‘field-of-view’ (FOV) of the satellite sensor after interacting with the ocean surface and the atmosphere.
The pathways are illustrated in FIG. 1. For example path (a) illustrates direct specular reflection of the sun's rays from the ocean surface; path (b) corresponds to direct scattering from the atmosphere alone, while path (c) represents rays that are scattered both by the ocean surface and by the atmosphere. The route followed in dashed path (d) is that taken by upwelling light backscattered by different oceanic layers that ultimately reach the ocean surface [2] and are further attenuated by the intervening atmosphere before arriving at the satellite detector [3]. The radiance component (d) carries useful information on the optical properties that are related to seawater constituents that color the ocean.
Most satellite sensors have the means of orienting the detector so as to avoid or minimize specular reflection of path (a) The atmospheric signal arising from path (b) constitutes nearly 80% of the signal scattered into the satellite detector, and carries useful information about the ocean. There are techniques which have been developed by Gordon &Wang (1994), Gordon (1997), and Gordon & Morel (1983) which deal with the removal of the atmospheric component. This will not be of concern to us here.
Hitherto known devices to measure general-angle scattering meter was by Tyler and Richardson (J. E. Tyler, W. H. Richardson, “Nephelometer for the measurement of volume scattering function in-situ”, J. Opt.Soc. Am 48, 354-357 (1958)). The first fixed angle optical backscatter devices were reported by Moore et al (C. A. Moore, R.Honey, D. hancock, S.Damron, and R.Hilbers, Development and use of computerized optical sea-truth instrumentation for LIDEX-82, SRI International Project 3878, Final Rep. 1984), and substantial improvements on these devices undertaken by R. Maffione and D. R. Dana (R. A. Maffione, D. R. Dana, “Instruments and methods for measuring the backward-scattering coefficient of ocean waters,” Applied Optics, Vol.36, No.24 pp 6057-6067, 1997) with the purpose of finding the best estimate of the back scattering coefficient (bb) via a measurement of Volume Scattering Function (VSF) at a fixed backscatter angle of 142°. The optical geometry of the backscatter meter developed by Maffione and Dana is illustrated in FIG. 2. The meter consists of built in collimated source of light [a light emitting diode—LED] having a finite divergence and propagated at a fixed refracted angle through a glass window of the instrument casing into surrounding seawater. The sample volume at which backscattering occurs (shown as a shaded area in FIG. 2) is the geometric intersection of the field-of-view [FOV] of the source and detector. The detector is placed adjacent to the source. The backscattered flux that falls within the solid angle cone subtended by the sample volume at the detector will be the signal measured by the detector. The receiver detector is frequency modulated so that it phase locks to only the backscattered flux from the LED source. This electro-optical technique rejects ambient light changes and permits its use in bright sunlight. The backscatter coefficient bb in this meter is computed on the basis of the Integral Mean value Theorem such that there is a fixed scattering angle ψ* of the VSF function where bb=2π.β(ψ*). The detected signal is proportional to β (ψ*) multiplied by a weighting function W (z;c) which varies with the distance z of the sample volume from the backscatter window (see FIG. 2), and the attenuation coefficient of seawater. Most backscatter devices are deployed from a ship or a boat. There are several known drawbacks to these optical backscatter devices which are identifiable:—                The small volume from which the backscattered flux originates may not capture a representative distribution of suspended particles in the ocean.        The detected signal has to calibrated by a separate experiment using a Lambertian target in clean water tank.        The beam attenuation coefficient [c] of the unknown water needs to be known so as to apply a correction to the measurement of backscattered flux. Further if [c] changes by a large factor, as happens in coastal waters, then the centroid angle (ψ*) at which the volume scattering function is measured will also change by several degrees.        
The present invention recognizes that the sun, the ocean and a satellite ocean color sensor constitute the elements of a giant backscatter sensor system in the sky. The source of radiation is the sun (the analogue to this is the light emitting diode as the exciting source of the backscatter meter), the ocean surface is the sample volume which emits backscattered flux from the top layer and all layers below the ocean surface in response to the stimulation provided by the sun's radiation, and the detector in our system is the satellite ocean color sensor which receives the backscattered flux from the ocean (the analogue to this is the single photodiode detector used in the backscatter meter).
The present invention provides the means to interpret the satellite signal in terms of the Volume Scattering Function [VSF] in the backward direction at fixed angles by using the existing geometry of a known sun-ocean-satellite detector system. Our best example is the Ocean Colour Monitor (OCM) on the Indian satellite platform IRS-P4, although the same ideas contained here could be applied to other Ocean Color Sensors namely SeaWiFs on the SeaStar satellite. The OCM operates as a push-broom camera which uses as detector, a charge coupled linear array device (CCD) as a light detector (see FIG. 3). The various pathways of photons (as depicted in FIG. 1) result from reflected and backscattered events in the atmosphere and from within the ocean's interior. These pathways are contained within the cone defined by a field of view of +43°, and a total swath width of ˜1420 kms. at sea level as seen by the satellite optics (FIG. 3 again). However, within this large cone, each element of the CCD array views an ocean pixel with infinitesimally narrow cones having fixed look angles and a IGFOV (Instantaneous Ground Field of View) of 360 m×236 m. There are 3730 active elements on the CCD array viewing an equal number of contiguous ocean pixels spanning the swath width of the satellite track. Each scan line in the push broom operation is contiguous with each other. Table 1 summarizes the major features of the OCM sensor as it exists on IRS-P4. The eight spectral channels on OCM have separate collecting optics and separate CCD line array detectors each having their own drive and signal processing electronics.
TABLE 1Features of OCM Camera (see reference 5)FeatureDescriptionOrbitPolar sun synchronousSatellite Altitude720 kmsSwath>1420 kmsRepeativity2 daysIGFOV (Ocean pixel)360 m × 236 mSpectral Channels (nm)414.2, 441.4, 485.7, 510.6, 556.4, 669,Bands 1-8768.6, 865.1Active CCD pixels3730 elements